The impending ability to manipulate electron spins using emerging technology and the need for ever-increasing magnetic storage density have driven researchers to search for novel ferromagnetic materials other than Fe, Co, Ni and their alloys. Remarkable phenomena, such as colossal magnetoresistance (CMR) and quantum-interference magnetoresistance have been discovered in these new materials. Unfortunately, most of these effects are only realized at low temperatures and in high magnetic fields, making potential applications impractical. For example, perovskite (or pyrochlore) manganites and related compounds exhibit CMR in the vicinity of the metal-insulator transition temperature. However, this only occurs below room temperature and under applied fields of several tesla.
Other novel materials, such as silver chalcogenides (Ag2+δSe, Ag2+δTe), CrO2, doped iron silicide (Fe1−xCoxSi, Fe1−xMnxSi), and inhomogeneous semiconductors such as Hg1−xCdxTe, also exhibit significant magnetoresistance (MR) only at low temperatures and/or in high fields. One interesting approach to reduce the field strength required to observe a large MR is to utilize tunneling MR (TMR) in either multilayer structures or granular (polycrystalline) materials.
Intergrain magnetoresistance (IMR) originates from the change of grain-to-grain electron tunneling in polycrystalline materials under applied magnetic fields. This effect becomes significant in materials where the spins of the itinerant electrons are highly polarized. In the so-called half metals, the carrier spins are completely polarized. These systems exhibit metallic transport behavior for one spin orientation and insulating behavior for the other. However, significant IMR only occurs below the Curie temperature (Tc), and thus room temperature IMR does not exist in Tl2Mn2O7 (Tc≈120° K.) and is nearly absent in perovskite manganites and CrO2.
Recent interest has been focused on the double perovskite material systems such as SrFe0.5Mo0.5O3, due to the large magnitude of the room temperature IMR. SrFe0.5Mo0.5O3 has an IMR of about 5% at room temperature in magnetic fields of about 10 kOe. The large fields required to achieve this level of IMR probably originate from the high saturation field in this material, and seriously limit the technological applications. For example, the field strength required for magnetic heads is in the range of a few hundred Oe. It is common practice to control magnetic softness by adjusting the magneto-elastic coupling through alloying. For example, the alloying of Ni with Fe significantly lowers the coercive and saturation fields. Furthermore, it has been well established that the substitution of different-size ions into the A-site of AMnO3 exerts a “chemical pressure” on the system. This chemical pressure causes pronounced changes in physical properties of the system such as TC and CMR.
Accordingly, it would be desirable to reduce the magnetic fields required for achieving the large magnitude room temperature IMRs in double perovskite material systems.